Abstract
Heavy damage was experienced during the Niigataken Chuetsu-oki earthquake in 2007 in the Kashiwazaki city of the Niigata prefecture. The area is covered with thick deep Tertiary sedimentary layers over the basement rock. In this study we conducted microtremor array explorations and receiver function analysis to reveal subsurface structure of the deep sediments in the Kashiwazaki city. The array observations of vertical microtremors were conducted at two sites in the northern and southern main parts of the Kashiwazaki city. Rayleigh-wave phase velocity at periods from 0.5 to 4.5 seconds was estimated from a frequency-wave number spectral analysis of the microtremors. We next analyzed the earthquake records observed at the Kashiwazaki station of the K-NET to derive a receiver function. Since the strong motion station is located near the northern array of the microtremor exploration, we conducted the joint inversion of the phase velocity and the receiver function to a 1D profile. The S-wave velocities of the inverted profile are 0.4, 0.7, 1.3, 2.4, and 3km/s. The depth to the top of the basement is about 5 km. The observed phase velocities and receiver function are well reconstructed with the theoretical ones for the inverted profile. Using the velocities of P- and S-waves, the phase velocities observed in the southern array of the microtremor exploration are inverted to a 1D profile to determine only the thickness of each layer. The basement depth is 4.3 km beneath the southern array. This is shallower than that for the northern array, indicating the dipping of the basement toward the south. We finally discussed site amplifications using the estimated profiles.
